Gallium nitride-based compound semiconductor light-emitting device and negative electrode thereof

ABSTRACT

An object of the present invention is to provide a negative electrode which attains excellent Ohmic contact with an n-type gallium nitride-based compound semiconductor layer and which resists deterioration in characteristics which would be caused by heating. Another object of the invention is to provide a gallium nitride-based compound semiconductor light-emitting device having the negative electrode. The inventive gallium nitride-based compound semiconductor light-emitting device comprises an n-type semiconductor layer of a gallium nitride-based compound semiconductor, a light-emitting layer of a gallium nitride-based compound semiconductor and a p-type semiconductor layer of a gallium nitride-based compound semiconductor formed on a substrate in this order, and has a negative electrode and a positive electrode provided on the n-type semiconductor layer and the p-type semiconductor layer, respectively; wherein the negative electrode comprises a bonding pad layer and a contact metal layer which is in contact with the n-type semiconductor layer, and the contact metal layer is composed of Cr or a Cr alloy and formed through sputtering.

CROSS REFERENCE TO RELATED APPLICATION

This application is an application filed under 35 U.S.C. §111(a)claiming benefit, pursuant to 35 U.S.C. §119(e)(1), of the filing dateof the Provisional Application No. 60/529,751 filed on Dec. 17, 2003,pursuant to 35 U.S.C. §111(b).

TECHNICAL FIELD

The present invention relates to a gallium nitride-based compoundsemiconductor light-emitting device, and more particularly to aflip-chip-type gallium nitride-based compound semiconductorlight-emitting device having a negative electrode that exhibitsexcellent characteristics and can be fabricated with high productivity.

BACKGROUND ART

In recent years, gallium nitride-based compound semiconductorsrepresented by the formula Al_(x)Ga_(y)In_(1-x-y)N (0≦x≦1, 0≦y≦1, x+y≦1)have become of interest as materials for producing a light-emittingdiode (LED) which emits ultraviolet to blue light, or green light.Through employment of such a compound semiconductor, ultraviolet light,blue light, or green light of high emission intensity can be obtained;such high-intensity light has conventionally been difficult to attain.Unlike the case of a GaAs light-emitting device, such a galliumnitride-based compound semiconductor is generally grown on a sapphiresubstrate (i.e., an insulating substrate); hence, an electrode cannot beprovided on the back surface of the substrate. Therefore, both anegative electrode and a positive electrode must be provided onsemiconductor layers formed through crystal growth on the substrate.

In the case of the gallium nitride-based compound semiconductor device,the sapphire substrate is permeable with respect to emitted light.Therefore, attention is drawn to a flip-chip-type light-emitting device,which is configured by mounting the semiconductor device on a lead frameand the like such that the electrode face the frame, whereby emittedlight is emitted through the sapphire substrate.

FIG. 1 is a schematic representation showing a general structure of aflip-chip-type light-emitting device. Specifically, the light-emittingdevice includes a substrate 1, a buffer layer 2, an n-type semiconductorlayer 3, a light-emitting layer 4, and a p-type semiconductor layer 5,the layers being formed atop the substrate through crystal growth. Aportion of the light-emitting layer 4 and a portion of the p-typesemiconductor layer 5 are removed through etching, thereby exposing aportion of the n-type semiconductor layer 3 to the outside. A positiveelectrode 10 is formed on the p-type semiconductor layer 5, and anegative electrode 20 is formed on the exposed portion of the n-typesemiconductor layer 3. The light-emitting device is mounted on, forexample, a lead frame such that the electrodes face the frame, followedby bonding.

During mounting of a flip-chip-type light-emitting device, a negativeelectrode is heated to some hundreds of degrees Celsius. Therefore, thenegative electrode of a flip-chip-type light-emitting device is requiredto resist a deterioration, in characteristics, caused by heating.

As an exemplary negative electrode which provides excellent Ohmiccontact with a gallium nitride-based compound semiconductor, there hasbeen known such an electrode that is formed through vapor deposition ofAl, Cr, and Ti on an n-type gallium nitride-based compound semiconductorlayer (see, for example, Japanese Patent Application Laid-Open (kokai)No. 5-291621). However, when heated, the negative electrode hasdeteriorated characteristics. Another known negative electrode is formedthrough vapor deposition, on an n-type gallium nitride-based compoundsemiconductor layer, of an undercoat layer formed of at least one metalselected from the group consisting of V, Nb, Zr, and Cr, or formed of analloy containing the metal and, on the undercoat layer, a main electrodeformed of a metal different from the metal forming the undercoat layer,followed by thermally annealing the formed multi-layer structure (see,for example, Japanese Patent Application Laid-Open (kokai) No.10-112555). However, as the above method includes a thermal annealingstep after formation of a negative electrode, productivity of theelectrode is unsatisfactory.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a negative electrodewhich attains excellent Ohmic contact with an n-type galliumnitride-based compound semiconductor layer and which resists adeterioration, in characteristics, which would be caused by heating.Another object of the invention is to provide a gallium nitride-basedcompound semiconductor light-emitting device having the negativeelectrode.

The present invention provides the following.

(1) A gallium nitride-based compound semiconductor light-emitting devicecomprising an n-type semiconductor layer of a gallium nitride-basedcompound semiconductor, a light-emitting layer of a galliumnitride-based compound semiconductor and a p-type semiconductor layer ofa gallium nitride-based compound semiconductor formed on a substrate inthis order, and having a negative electrode and a positive electrodeprovided on the n-type semiconductor layer and the p-type semiconductorlayer, respectively; wherein the negative electrode comprises a bondingpad layer and a contact metal layer which is in contact with the n-typesemiconductor layer, and the contact metal layer is composed of Cr or aCr alloy and formed through sputtering.

(2) A gallium nitride-based compound semiconductor light-emitting deviceas described in (1) above, wherein the Cr alloy includes Cr and ametallic element having a work function of 4.5 eV or less.

(3) A gallium nitride-based compound semiconductor light-emitting deviceas described in (2) above, wherein the metallic element having a workfunction of 4.5 eV or less is at least one metallic element selectedfrom the group consisting of Al, Ti, Si, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo,Hf, Ta, W, and V.

(4) A gallium nitride-based compound semiconductor light-emitting deviceas described in (2) above, wherein the metallic element having a workfunction of 4.5 eV or less is at least one metallic element selectedfrom the group consisting of Al, V, Nb, Mo, W, and Mn.

(5) A gallium nitride-based compound semiconductor light-emitting deviceas described in any one of (1) to (4) above, wherein the Cr alloy has aCr content of 1 mass % or more and less than 100 mass %.

(6) A gallium nitride-based compound semiconductor light-emitting deviceas described in (5) above, wherein the Cr alloy has a Cr content of 10mass % or more.

(7) A gallium nitride-based compound semiconductor light-emitting deviceas described in any one of (1) to (6) above, wherein the contact metallayer has a thickness of 1 to 500 nm.

(8) A gallium nitride-based compound semiconductor light-emitting deviceas described in (7) above, wherein the contact metal layer has athickness of 10 nm or more.

(9) A gallium nitride-based compound semiconductor light-emitting deviceas described in any one of (1) to (8) above, wherein the bonding padlayer is formed of a metal selected from the group consisting of Au, Al,Ni, and Cu, or an alloy containing the metal.

(10) A gallium nitride-based compound semiconductor light-emittingdevice as described in any one of (1) to (9) above, wherein the bondingpad layer has a thickness of 100 to 1,000 nm.

(11) A gallium nitride-based compound semiconductor light-emittingdevice as described in (10) above, wherein the bonding pad layer has athickness of 200 to 500 nm.

(12) A gallium nitride-based compound semiconductor light-emittingdevice as described in any one of (1) to (11) above, wherein an Au—Snalloy layer is provided on the bonding pad layer.

(13) A gallium nitride-based compound semiconductor light-emittingdevice as described in (12) above, wherein the Au—Sn alloy layer has athickness of 200 nm or more.

(14) A gallium nitride-based compound semiconductor light-emittingdevice as described in any one of (1) to (11) above, wherein a lead freesolder layer is provided on the bonding pad layer.

(15) A gallium nitride-based compound semiconductor light-emittingdevice as described in (14) above, wherein the lead free solder layerhas a thickness of 200 nm or more.

(16) A gallium nitride-based compound semiconductor light-emittingdevice as described in any one of (1) to (15) above, wherein thelight-emitting device has an adhesion layer formed of Ti between thecontact metal layer and the bonding pad layer.

(17) A gallium nitride-based compound semiconductor light-emittingdevice as described in (16) above, wherein the adhesion layer has athickness of 1 to 100 nm.

(18) A gallium nitride-based compound semiconductor light-emittingdevice as described in (17) above, wherein the adhesion layer has athickness of 10 nm or more.

(19) A gallium nitride-based compound semiconductor light-emittingdevice as described in any one of (1) to (15) above, wherein thelight-emitting device has a barrier layer between the contact metallayer and the bonding pad layer.

(20) A gallium nitride-based compound semiconductor light-emittingdevice as described in any one of (12) to (18) above, wherein thelight-emitting device has a barrier layer between the bonding pad layerand the Au—Sn alloy layer or the lead free solder layer.

(21) A gallium nitride-based compound semiconductor light-emittingdevice as described in (19) or (20) above, wherein the barrier layer isformed of a metal selected from the group consisting of Ti, Zr, Hf, Ta,W, Re, Os, Ir, Pt, Fe, Co, Ni, Ru, Rh, and Pd, or an alloy containingthe metal.

(22) A gallium nitride-based compound semiconductor light-emittingdevice as described in (21) above, wherein the barrier layer is formedof a metal selected from the group consisting of Ti, Ta, W, and Pt, oran alloy containing the metal.

(23) A gallium nitride-based compound semiconductor light-emittingdevice as described in any one of (19) to (22) above, wherein thebarrier layer has a thickness of 10 to 500 nm.

(24) A gallium nitride-based compound semiconductor light-emittingdevice as described in (23) above, wherein the barrier layer has athickness of 50 to 300 nm.

(25) A gallium nitride-based compound semiconductor light-emittingdevice as described in any one of (1) to (24) above, wherein thelight-emitting device is of a flip-chip type.

(26) A negative electrode for use in a gallium nitride-based compoundsemiconductor light-emitting device comprising a bonding pad layer and acontact metal layer which is in contact with the n-type semiconductorlayer, wherein the contact metal layer is composed of Cr or a Cr alloyand formed through sputtering.

(27) A negative electrode for use in a gallium nitride-based compoundsemiconductor light-emitting device as described in (26), wherein thelight-emitting device is of a flip-chip type.

(28) A method for manufacturing a gallium nitride-based compoundsemiconductor light-emitting device comprising

(a) forming an n-type semiconductor layer of a gallium nitride-basedcompound semiconductor, a light-emitting layer of a galliumnitride-based compound semiconductor and a p-type semiconductor layer ofa gallium nitride-based compound semiconductor on a substrate in thisorder,

(b) providing a positive electrode and a negative electrode, whichcomprises a bonding pad layer and a contact metal layer, on the p-typesemiconductor layer and the n-type semiconductor layer, respectively;wherein the contact metal layer is forming through sputtering Cr or a Cralloy on the n-type semiconductor layer to attain Ohmic contact withoutperforming an annealing.

(29) A lamp comprising a gallium nitride compound semiconductorlight-emitting device as described in any one of (1) to (25).

The negative electrode according to the present invention has a contactmetal layer comprising Cr or a Cr alloy and formed through sputtering.Thus, the negative electrode attains excellent Ohmic contact with ann-type gallium nitride-based compound semiconductor layer and is notdeteriorated, in characteristics, by heating. Such excellent Ohmiccontact between the negative electrode of the present invention and then-type gallium nitride-based compound semiconductor layer can beprovided without performing annealing. Therefore, the galliumnitride-based compound semiconductor light-emitting device of thepresent invention can be produced with remarkably high efficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing a general structure of a conventionalflip-chip-type compound semiconductor light-emitting device.

FIG. 2 is a schematic view showing an exemplary flip-chip-type galliumnitride-based compound semiconductor light-emitting device according tothe present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

In the present invention, no particular limitations are imposed on thegallium nitride-based compound semiconductor layers stacked on asubstrate, and the semiconductor stacked layers may have aconventionally known structure as shown in FIG. 1; i.e., a stackedstructure including a buffer layer 2, an n-type semiconductor layer 3, alight-emitting layer 4, and a p-type semiconductor layer 5, the layersbeing formed atop a substrate 1 through crystal growth. No particularlimitation is imposed on the type of the substrate, and anyconventionally known substrates such as a sapphire substrate and an SiCsubstrate may be employed. No particular limitation is imposed on thetype of the gallium nitride-based compound semiconductor, andconventionally known gallium nitride-based compound semiconductorsrepresented by formula: Al_(x)Ga_(y)In_(1-x-y)N (0≦x≦1, 0≦y≦1, x+y≦1)may be employed.

FIG. 2 shows one exemplary employable stacked structure in which an AlNbuffer layer 2, an n-type GaN contact layer 3 a, an n-type GaN lowercladding layer 3 b, an InGaN light-emitting layer 4, a p-type AlGaNupper cladding layer 5 b, and a p-type GaN contact layer 5 a aresequentially stacked on a sapphire substrate 1. No particular limitationis imposed on the composition and structure of a positive electrode 10to be provided atop the contact layer 5 a, and a positive electrodehaving a conventional composition and structure (e.g., Al) may beemployed.

A portion of the contact layer 5 a, that of the upper cladding layer 5b, that of the light-emitting layer 4, and that of the lower claddinglayer 3 b, these layers being formed of the aforementioned galliumnitride-based compound semiconductor, are removed through etching, and anegative electrode 20 is provided on the thus-exposed portion of thecontact layer 3 a. The negative electrode 20 is constituted of a contactmetal layer 21, an adhesion layer 22, and a bonding pad layer 23.

According to the present invention, the negative electrode at leastcontains two layers; i.e., a contact metal layer which is in Ohmiccontact with an n-type semiconductor layer, and a bonding pad layer forestablishing electric contact with a circuit substrate, a lead frame,etc., and the contact metal layer is composed of Cr or a Cr alloy andformed through sputtering. As formation of the contact metal layerthrough sputtering promotes alloying of the n-type GaN layer with thecontact metal layer, a low contact resistance can be attained afterformation of the contact metal layer without carrying out annealing.

The contact metal layer preferably has a thickness of 1 nm or more. Alayer thickness of 5 nm or more is particularly preferred, from theviewpoint of attainment of low resistance. The thickness is morepreferably 10 nm or more, as a constant low resistance can then beattained. However, when the thickness is excessively large, productivitydecreases. Thus, the thickness is preferably 500 nm or less, morepreferably 200 nm or less.

The Cr alloy is preferably an Cr alloy containing a metallic elementhaving a work function of 4.5 eV or less, since such a Cr alloy has lowspecific contact resistance. Examples of the metallic element having awork function of 4.5 eV or less include Al, Ti, Si, Mn, Fe, Co, Ni, Cu,Zr, Nb, Mo, Hf, Ta, W, and V. Among them, Al, V, Nb, Mo, W, and Mn areparticularly preferred.

The Cr content of the Cr alloy is preferably 1 mass % or more and lessthan 100 mass %. Particularly when thermal-induced deterioration incharacteristics of the light-emitting device is to be prevented, the Crcontent is preferably controlled to 10 mass % or more. In order to morereliably prevent deterioration in characteristics, the Cr content ispreferably controlled to 20 mass % or more. Also, as a reflectivity ofthe Cr alloy becomes higher after heating, it is preferable that the Crcontent is high.

Sputtering may be performed by means of a conventionally knownsputtering apparatus under appropriately selected conditions which areknown. Specifically, gallium nitride-based compound semiconductor layersare stacked on a substrate, and a portion of an n-type semiconductorlayer is exposed through etching. The thus processed substrate is placedin a chamber, and the substrate temperature is predetermined so as tofall within a range of room temperature to 500° C. The substratetemperature is preferably controlled to about room temperature. Thechamber is evacuated until the degree of vacuum is 10⁻⁴ to 10⁻⁷ Pa.Examples of employable sputtering gas include He, Ne, Ar, Kr, and Xe.Among them, Ar is preferred, from the viewpoint of availability. Any oneof these gases is introduced into the chamber, and the internal pressureis adjusted to 0.1 to 10 Pa. Under these conditions, discharging isperformed. The internal pressure is preferably adjusted to 0.2 to 5 Pa.The electric power input to the apparatus preferably falls within arange of 0.2 to 2.0 kW. During sputtering, the thickness of the formedlayer can be controlled by tuning discharge time and input electricpower. The sputtering target to be employed preferably has an oxygencontent of 10,000 ppm or less, as the oxygen content of the formed layerdecreases. The oxygen content of the sputtering target is morepreferably 6,000 ppm or less. When an alloy layer is formed throughsputtering, in a preferred manner, an alloy having a target compositionis prepared and formed into a sputtering target. The sputtering targethaving a target composition identical to that of the alloy layer issputtered.

The contact resistance between the contact metal layer and an n-type GaNsemiconductor layer is greatly affected by the degree of removal ofoxide film formed spontaneously on the surface of the n-type GaNsemiconductor layer. The surface of the GaN semiconductor layer isoxidized in the atmosphere, thereby forming natural oxide film. Even inthe case where the thus-formed oxide film is removed through etching orsimilar means, the surface is oxidized again, if the surface is exposedto the atmosphere before formation of an electrode. Since the oxide filmformed on GaN serves as an insulator, the contact resistance at theinterface between the electrode and GaN increases, if the entire GaNsurface is covered with the oxide film. Therefore, removal of oxide filmformed on the surface of an n-type semiconductor layer before sputteringis a key issue.

The bonding pad layer is preferably formed of a metal selected from thegroup consisting of Au, Al, Ni, and Cu, or formed of an alloy containingthe metal, from the viewpoint of attainment of good contact with a bump.The bonding pad layer preferably has a thickness of 100 to 1,000 nm,from the viewpoint of productivity. The thickness is preferably 200 to800 nm, particularly preferably 200 to 500 nm.

In order to improve adhesion between the contact metal layer and thebonding pad layer, an adhesion layer formed of Ti preferably intervenesbetween the two layers. The adhesion layer, if employed, preferably hasa thickness of 1 to 100 nm. When the thickness is less than 1 nm, theeffect of adhesion is poor, whereas when the thickness is more than 100nm, the Ti film is oxidized in the case where the light-emitting deviceis in a heated state, and in some cases, the electrical characteristicsmay be impaired. From the viewpoint of reliable adhesion effect, athickness of 5 nm or more is preferred, with 10 nm or more beingparticularly preferred.

An Au—Sn alloy layer or a lead free solder layer can be preferablyprovided on the bonding pad layer. This layer functions as an adhesionlayer in order to adhere the light-emitting device to a sub-mount. Thislayer preferably has a thickness of 200 nm or more, from the viewpointof attaining the adhesion. Also, this layer preferably has a thicknessof 5 μm or less, from the viewpoint of productivity.

Even in a case of providing the Au—Sn alloy layer or the lead freesolder layer, a negative electrode is heated to 300˜400° C. for someminutes during mounting. By the heat generated during mounting, a Cratom in the contact metal layer may diffuse to the bonding pad layer andthe Au—Sn alloy layer or the lead free solder layer.

Therefore, in order to prevent the Cr diffusion, a barrier layer can bepreferably provided between the contact metal layer and the bonding padlayer or between the bonding pad layer and the Au—Sn alloy layer or thelead free solder layer. The barrier layer is preferably formed of ametal selected from the group consisting of Ti, Zr, Hf, Ta, W, Re, Os,Ir, Pt, Fe, Co, Ni, Ru, Rh, and Pd, or formed of an alloy containing themetal. Among these metals, Ti, Ta, W, and Pt are more preferable. Thebarrier layer preferably has a thickness of 10 nm or more, from theviewpoint of forming a uniform single layer. Also, it preferably has athickness of 500 nm or less, from the viewpoint of productivity. Thethickness is more preferably 50 to 300 nm.

When the above-mentioned adhesion layer of Ti is provided, it alsofunctions as the barrier layer.

No particular limitation is imposed on the method of forming the bondingpad layer, the adhesion layer and the barrier layer, and anyconventionally known method such as sputtering or vapor deposition maybe employed. However, as the contact metal layer is formed throughsputtering, preferably, these two layers are subsequently formed alsothrough sputtering for the purpose of simplifying the steps. Also, noparticular limitation is imposed on the method of forming the Au—Snalloy layer or the lead free solder layer, and any conventionally knownmethod such as vapor deposition, plating and a coating method using apaste, may be employed.

The present inventive gallium nitride-based compound semiconductorlight-emitting device can preferably form a lamp.

EXAMPLES

The present invention will next be described in more detail by way ofExamples and Comparative Examples. Table 1 shows negative electrodematerials employed in the Examples and Comparative Examples, theelectrode formation method, and evaluation of characteristicsimmediately after formation of film and after a heating test. Needlessto say, these Examples and Comparative Examples should not be construedas limiting the invention.

FIG. 2 is a schematic view showing a gallium nitride-based compoundsemiconductor light-emitting device produced in the present Example.

The gallium nitride-based compound semiconductor stacked structureemployed for fabricating the light-emitting device was produced throughthe following procedure: an AlN buffer layer 2 was formed on a sapphiresubstrate 1; and an n-type GaN contact layer 3 a, an n-type GaN lowercladding layer 3 b, an InGaN light-emitting layer 4, a p-type AlGaNupper cladding layer 5 b, and a p-type GaN contact layer 5 a weresuccessively formed atop the buffer layer 2. The contact layer 3 a isformed of n-type GaN doped with Si (7×10¹⁸/cm³), the lower claddinglayer 3 b is formed of n-type GaN doped with Si (5×10¹⁸/cm³), and thelight-emitting layer 4, having a single quantum well structure, isformed of In_(0.95)Ga_(0.05)N. The upper cladding layer 5 b is formed ofp-type Al_(0.25)Ga_(0.75)N doped with Mg (1×10¹⁸/cm³). The contact layer5 a is formed of p-type GaN doped with Mg (5×10⁹/cm³). Stacking of theselayers was performed by means of MOCVD under typical conditions whichare well known in the art.

A positive electrode 10 and a negative electrode 20 were provided on thegallium nitride-based compound semiconductor stacked structure throughthe below-described procedure, to thereby fabricate a flip-chip-typegallium nitride-based compound semiconductor light-emitting device.

Firstly, in order to remove the oxide film on the contact layer 5 a, thegallium nitride-based compound semiconductor stacked structure wastreated in boiling concentrated HCl for 10 minutes.

Then, the positive electrode 10 made of Al was formed on the contactlayer 5 a through the following procedure. A resist was uniformlyapplied onto the entire surface of the contact layer, and a portion ofthe resist provided on the region where the positive electrode was to beformed was removed through a conventional lithographic technique. Thethus-formed structure was immersed in buffered hydrofluoric acid (BHF)at room temperature for one minute, followed by forming a positiveelectrode in the form of film in a vacuum sputtering apparatus. Theoperational conditions employed for forming the electrode throughsputtering are as follows.

The chamber was evacuated to a degree of vacuum of 10⁻⁶ Pa. A substratewas placed in the chamber, and Ar serving as a sputtering gas was fedinto the chamber. After the internal pressure of the chamber wascontrolled to 0.5 Pa, discharge was caused by inputting an electricpower of 0.25 kW. The thickness of the formed layer was adjusted to 100nm by tuning the discharge time and supplied electric power. Thestructure was removed from the sputtering apparatus, and a portion ofmetallic film other than the positive electrode region was removed alongwith the resist through a lift-off technique.

Subsequently, a negative electrode was formed on the contact layer 3 athrough the following procedure.

Firstly, an etching mask was formed in the positive electrode throughthe following procedure. After uniform provision of a resist on theentire surface, a portion of the resist corresponding to a regionslightly wider than the positive electrode region was removed through aconventional lithography technique. The structure was placed in a vacuumvapor deposition apparatus, and an Ni layer and a Ti layer were stacked,through the electron beam method, to thicknesses of about 50 nm and 300nm, respectively, under a pressure of 4×10⁻⁴ Pa or lower. Thereafter, aportion of metal film other than the positive electrode region wasremoved along with the resist through the lift-off technique. Theetching mask serves as a protective layer for protecting the positiveelectrode from plasma-induced damage during reactive ion dry etching.

Subsequently, the contact layer 3 a was exposed, and a negativeelectrode was formed on the thus-exposed portion through the followingprocedure. Specifically, the semiconductor stacked structure was etchedthrough reactive ion dry etching until the contact layer 3 a wasexposed, and the resultant stacked structure was removed from the dryetching apparatus. The aforementioned etching mask was removed by use ofnitric acid or hydrofluoric acid.

After uniform provision of a resist on the entire surface, a portion ofthe resist corresponding to the exposed contact layer 3 a region wasremoved through a conventional lithography technique. Subsequently,through the aforementioned sputtering method, a Cr contact metal layer21, a Ti adhesion layer 22, and an Au bonding pad layer 23 were formedat a thickness of 100 nm, 20 nm, and 300 nm, respectively. Thereafter, aportion of metal film other than the negative electrode region wasremoved along with the resist, thereby fabricating the galliumnitride-based compound semiconductor light-emitting device of thepresent invention.

In a similar manner, gallium nitride-based compound semiconductorlight-emitting devices were fabricated by use of contact metal layermaterials, adhesion layer materials, and bonding pad layer materialslisted in Table 1. Specific contact resistance values of thethus-produced light-emitting devices were determined through thecircular TLM method. The results are also shown in Table 1.

TABLE 1

TABLE 1 After film formation After heating Specific Specific Contactcontact contact metal Adhesion Bonding Film resistance resistance layerlayer pad layer formation Type Ω · cm² Type Ω · cm² Cr — Al VD Ohmic 4 ×10⁻⁵ Schottky Cr — Au VD Ohmic 3 × 10⁻⁵ Schottky Cr Ti Au VD Ohmic 3 ×10⁻⁵ Schottky Cr — Al SPT Ohmic 4 × 10⁻⁵ Ohmic 3 × 10⁻⁵ Cr — Au SPTOhmic 3 × 10⁻⁵ Ohmic 3 × 10⁻⁵ Cr Ti Au SPT Ohmic 3 × 10⁻⁵ Ohmic 3 × 10⁻⁵Cr₂₀Al₈₀ Ti Au SPT Ohmic 4 × 10⁻⁵ Ohmic 3 × 10⁻⁵ Cr₈₀Al₂₀ Ti Au SPTOhmic 4 × 10⁻⁵ Ohmic 3 × 10⁻⁵ Al Ti Au SPT Ohmic 3 × 10⁻⁵ Ohmic 4 × 10⁻⁵Cr₅₀Ti₅₀ Ti Au SPT Ohmic 2 × 10⁻⁵ Ohmic 3 × 10⁻⁵ Cr₅₀Si₅₀ Ti Au SPTOhmic 4 × 10⁻⁵ Ohmic 3 × 10⁻⁵ Cr₅₀Mn₅₀ Ti Au SPT Ohmic 4 × 10⁻⁵ Ohmic 4× 10⁻⁵ Cr₅₀Fe₅₀ Ti Au SPT Ohmic 5 × 10⁻⁵ Ohmic 4 × 10⁻⁵ Cr₅₀Co₅₀ Ti AuSPT Ohmic 5 × 10⁻⁵ Ohmic 5 × 10⁻⁵ Cr₅₀Ni₅₀ Ti Al SPT Ohmic 5 × 10⁻⁵Ohmic 5 × 10⁻⁵ Cr₅₀Cu₅₀ Ti Au SPT Ohmic 6 × 10⁻⁵ Ohmic 6 × 10⁻⁵ Cr₅₀Zr₅₀Ti Au SPT Ohmic 3 × 10⁻⁵ Ohmic 4 × 10⁻⁵ Cr₅₀Nb₅₀ Ti Au SPT Ohmic 3 ×10⁻⁵ Ohmic 3 × 10⁻⁵ Cr₅₀Mo₅₀ Ti Au SPT Ohmic 2 × 10⁻⁵ Ohmic 2 × 10⁻⁵Cr₅₀Hf₅₀ Ti Au SPT Ohmic 4 × 10⁻⁵ Ohmic 3 × 10⁻⁵ Cr₅₀Ta₅₀ Ti Au SPTOhmic 4 × 10⁻⁵ Ohmic 3 × 10⁻⁵ Cr₅₀W₅₀ Ti Au SPT Ohmic 4 × 10⁻⁵ Ohmic 3 ×10⁻⁵

In the film formation method in Table 1, “VD” refers to a vapordeposition method for forming a comparative negative electrode. Thecomparative electrode was formed by placing a substrate in a furnace anddepositing a metal through a vapor deposition method in a vacuum of 10⁻⁴Pa or lower. The “SPT” refers to the sputtering method. The heating testwas performed in a RTA furnace in the atmosphere at 300° C. for oneminute.

When the contact metal layer is formed through vapor deposition, theas-formed film exhibits Ohmic contact characteristics. However, afterthe heating test has been performed, the film exhibits Schottky contactcharacteristics, and the resistance value is considerably impaired. Incontrast, according to the present invention, the film formed throughsputtering exhibits Ohmic contact characteristics after formation of thefilm and after performance of the heating test, and the resistance valueremains at a low level. When the contact metal layer is formed throughsputtering by use of a Cr—Al alloy target, the excellent characteristicsare maintained even at a Cr content of as low as 20%. Through employmentof a Cr—Mo alloy, the resistance value further decreases, and such a lowresistance value is maintained even after performance of the heatingtest.

INDUSTRIAL APPLICABILITY

The flip-chip-type gallium nitride-based compound semiconductorlight-emitting device provided according to the present invention isuseful for fabricating light-emitting diodes, lamps, etc.

1. A gallium nitride-based compound semiconductor light-emitting devicecomprising an n-type semiconductor layer of a gallium nitride-basedcompound semiconductor, a light-emitting layer of a galliumnitride-based compound semiconductor and a p-type semiconductor layer ofa gallium nitride-based compound semiconductor formed on a substrate inthis order, and having a negative electrode and a positive electrodeprovided on the n-type semiconductor layer and the p-type semiconductorlayer, respectively; wherein the negative electrode comprises a bondingpad layer and a contact metal layer which is in contact with the n-typesemiconductor layer, and the contact metal layer is composed of Cr or aCr alloy and formed through sputtering.
 2. A gallium nitride-basedcompound semiconductor light-emitting device according to claim 1,wherein the Cr alloy includes Cr and a metallic element having a workfunction of 4.5 eV or less.
 3. A gallium nitride-based compoundsemiconductor light-emitting device according to claim 2, wherein themetallic element having a work function of 4.5 eV or less is at leastone metallic element selected from the group consisting of Al, Ti, Si,Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Hf, Ta, W, and V.
 4. A galliumnitride-based compound semiconductor light-emitting device according toclaim 3, wherein the metallic element having a work function of 4.5 eVor less is at least one metallic element selected from the groupconsisting of Al, V, Nb, Mo, W, and Mn.
 5. A gallium nitride-basedcompound semiconductor light-emitting device according to claim 1,wherein the Cr alloy has a Cr content of 1 mass % or more and less than100 mass %.
 6. A gallium nitride-based compound semiconductorlight-emitting device according to claim 5, wherein the Cr alloy has aCr content of 10 mass % or more.
 7. A gallium nitride-based compoundsemiconductor light-emitting device according to claim 1, wherein thecontact metal layer has a thickness of 1 to 500 nm.
 8. A galliumnitride-based compound semiconductor light-emitting device according toclaim 7, wherein the contact metal layer has a thickness of 10 nm ormore.
 9. A gallium nitride-based compound semiconductor light-emittingdevice according to claim 1, wherein the bonding pad layer is formed ofa metal selected from the group consisting of Au, Al, Ni, and Cu, or analloy containing the metal.
 10. A gallium nitride-based compoundsemiconductor light-emitting device according to claim 1, wherein thebonding pad layer has a thickness of 100 to 1,000 nm.
 11. A galliumnitride-based compound semiconductor light-emitting device according toclaim 10, wherein the bonding pad layer has a thickness of 200 to 500nm.
 12. A gallium nitride-based compound semiconductor light-emittingdevice according to claim 1, wherein an Au—Sn alloy layer is provided onthe bonding pad layer.
 13. A gallium nitride-based compoundsemiconductor light-emitting device according to claim 12, wherein theAu—Sn alloy layer has a thickness of 200 nm or more.
 14. A galliumnitride-based compound semiconductor light-emitting device according toclaim 1, wherein a lead free solder layer is provided on the bonding padlayer.
 15. A gallium nitride-based compound semiconductor light-emittingdevice according to claim 14, wherein the lead free solder layer has athickness of 200 nm or more.
 16. A gallium nitride-based compoundsemiconductor light-emitting device according to claim 1, wherein thelight-emitting device has an adhesion layer formed of Ti between thecontact metal layer and the bonding pad layer.
 17. A galliumnitride-based compound semiconductor light-emitting device according toclaim 16, wherein the adhesion layer has a thickness of 1 to 100 nm. 18.A gallium nitride-based compound semiconductor light-emitting deviceaccording to claim 17, wherein the adhesion layer has a thickness of 10nm or more.
 19. A gallium nitride-based compound semiconductorlight-emitting device according to claim 1, wherein the light-emittingdevice has a barrier layer between the contact metal layer and thebonding pad layer.
 20. A gallium nitride-based compound semiconductorlight-emitting device according to claim 12, wherein the light-emittingdevice has a barrier layer between the bonding pad layer and the Au—Snalloy layer or the lead free solder layer.
 21. A gallium nitride-basedcompound semiconductor light-emitting device according to claim 19,wherein the barrier layer is formed of a metal selected from the groupconsisting of Ti, Zr, Hf, Ta, W, Re, Os, Ir, Pt, Fe, Co, Ni, Ru, Rh, andPd, or an alloy containing the metal.
 22. A gallium nitride-basedcompound semiconductor light-emitting device according to claim 21,wherein the barrier layer is formed of a metal selected from the groupconsisting of Ti, Ta, W, and Pt, or an alloy containing the metal.
 23. Agallium nitride-based compound semiconductor light-emitting deviceaccording to claim 19, wherein the barrier layer has a thickness of 10to 500 nm.
 24. A gallium nitride-based compound semiconductorlight-emitting device according to claim 23, wherein the barrier layerhas a thickness of 50 to 300 nm.
 25. A gallium nitride-based compoundsemiconductor light-emitting device according to claim 1, wherein thelight-emitting device is of a flip-chip type.
 26. A negative electrodefor use in a gallium nitride-based compound semiconductor light-emittingdevice comprising a bonding pad layer and a contact metal layer which isin contact with the n-type semiconductor layer, wherein the contactmetal layer is composed of Cr or a Cr alloy and formed throughsputtering.
 27. A negative electrode for use in a gallium nitride-basedcompound semiconductor light-emitting device according to claim 26,wherein the light-emitting device is of a flip-chip type.
 28. A methodfor manufacturing a gallium nitride-based compound semiconductorlight-emitting device comprising (a) forming an n-type semiconductorlayer of a gallium nitride-based compound semiconductor, alight-emitting layer of a gallium nitride-based compound semiconductorand a p-type semiconductor layer of a gallium nitride-based compoundsemiconductor on a substrate in this order, (b) providing a positiveelectrode and a negative electrode, which comprises a bonding pad layerand a contact metal layer, on the p-type semiconductor layer and then-type semiconductor layer, respectively; wherein the contact metallayer is forming through sputtering Cr or a Cr alloy on the n-typesemiconductor layer to attain Ohmic contact without performingannealing.
 29. A lamp comprising a gallium nitride compoundsemiconductor light-emitting device according to claim 1.